Application Express :: How To Update Total - New Value Doesn't Display In Field?

Let's say that we have an order entry app and the price and quantity items are already populated. We now want to show the user the extended price (the product of the first two items). the extended price would automatically display after both the quantity and price items are entered. I'm assuming that would require some kind of client-side processing. I imagine server-side processing is simpler, so I'll lean in that direction. I'm sure we've all seen web sites that require you to click a button for the order totals to be updated. I'm okay with that approach, but I haven't quite put all of the pieces together. Here's what I came up with.

A region button was added to trigger the updating of the extended price. The button's "Action When Button Clicked" is to submit the page. Under "Page Processing", a process was created to do the calculation with the recipient of the product being a page item. From debug code in the procedure, I can see that the item containing the extended price was, indeed, modified. But the new value doesn't display in the field and the Session window shows that the value of the item didn't change.

I must be missing something. The debug code says the item value was updated. The Session window says it wasn't updated. What gives? What did I do wrong and what needs to be done to correct this?